Are these 5% and 2% mass-volume?
To dilute a 5% solution to a 2% solution, we need to dilute it by a factor of 2.5 (2/5 = 1/2.5).
So a 5% solution contains 5 g HCl/100 mL solution. So if we end up with 1000 mL of solution, how many g HCl must be there to be 2%. X/1000 = 0.02 = 2,000 g. With a solution that is 5 g/100, how much must we use to obtain 2,000 g? That will be 2000/5 = 400. So we take 400 g of the 5% solution, make to 1000 mL, and we have a 2% solution.
See if you can come up with some helpful hints for the other posts that concern concn.

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